# 019 — Default port should be 0

## Summary

`--port` currently defaults to `8420` (`Args` in `main.rs`). The wishlist asks for a
default of `0`, i.e. let the OS assign an ephemeral free port rather than colliding
with whatever else is running on a fixed port.

## Scope

- Change `Args::port`'s default to `0`.
- After `TcpListener::bind` succeeds, read back the actual bound port (via
  `listener.local_addr()`) since binding to `0` means the requested port and the
  real one differ.
- Print the real bound port in the startup log line (it already prints
  `args.port`, which would otherwise always read `0`).
- Consider whether the actual port needs to be surfaced anywhere else (e.g. a
  `GET /api/stats`-style endpoint) — not required unless something already depends
  on knowing it programmatically today; check before adding.

## Acceptance criteria

- Running `ottyl` with no `--port` binds an OS-assigned free port instead of a fixed
  one, and the startup message prints that actual port.
- `--port <n>` still binds the requested fixed port as before.

## Depends on

None.